In the related art, there is known a batteryless absolute encoder including a plurality of absolute position detectors connected to a plurality of driven rotary shafts coupled to a main spindle via a gear mechanism to detect the respective rotational angles of the driven rotary shafts in order to detect the absolute position of the main spindle. For example, Japanese Unexamined Patent Application Publication No. 61-4918 (JP61-4918A) discloses a batteryless absolute encoder for detecting the absolute position of a spindle over a large number of revolutions using three resolvers coupled through a gear mechanism.
Japanese Patent No. 3704462 (JP3704462) discloses a batteryless absolute encoder for detecting the absolute position, including the number of turns, of a spindle on the basis of the positional relationship between four reluctance resolvers coupled through a gear mechanism. In such an encoder according to the related art, a combination of gears are chosen to increase the detectable range in order to cover a range of portions of a revolution that are necessary for external output. The number of revolutions can be efficiently calculated by an arrangement in which the respective numbers of teeth of adjacent gears are prime numbers with respect to each other. One of the resolvers is used to both determine which portion of a revolution the spindle is at and determine the number of revolutions of the spindle. The plurality of gears are disposed in two planes, and are disposed on the inner circumferential surface of one of the resolvers with a large diameter in order to achieve a flat configuration.